The present invention relates to a light measuring apparatus adapted to measure the intensity of light being measured and suitably usable as a spectral luminometer or a spectral calorimeter and a method for correcting the non-linearity of such a light measuring apparatus.
A polychrometer for simultaneously measuring spectral intensities of all the wavelengths in a measurement wavelength range is widely used as the one of a spectral luminometer (spectral intensity measuring apparatus) for measuring a spectral intensity distribution of light being measured or as the one of a spectral calorimeter (spectral reflection characteristic measuring apparatus) for measuring a spectral reflection characteristic of a sample due to its high measurement efficiency, its capability of measuring an instant light and other factors. FIG. 16 is a sectional view showing a schematic construction of a polychrometer 1 generally in use. The polychrometer 1 includes an incidence opening 3 formed in a housing 2, a light receiving sensor array 4, a diffraction grating 5 and a focusing optical system 6 for forming a wavelength-dispersed image of the incidence opening 3 by a beam having passed the incidence opening 3 on the light receiving sensor array 4. An output from the light receiving sensor array 4 is used to obtain the spectral intensity distribution or the spectral reflection characteristic by a calculation controlling circuit 8 after being subjected to a current-to-voltage conversion in a signal processing circuit 7.
FIG. 17 is a block diagram showing an exemplary construction of the signal processing circuit 7. The light receiving sensor array 4 is an array of (n) photodiodes and output currents of the respective photodiodes d1 to dn are converted into voltage values and amplified in accordance with a high gain by individually provided amplifiers a11 to a1n. In the following description, suffixes 1 to n are attached in the case of showing the constructions of n channels corresponding to the photodiode array comprised of (n) pixels, whereas these suffixes are not attached in the case that it is not necessary to particularly specify the channels. Outputs of the respective amplifiers a11 to a1n are inputted to a multiplexer 9, which successively selects and outputs the outputs from the respective amplifiers a11 to a1n in response to a switch signal from the calculation controlling circuit 8. The output of the multiplexer 9 is amplified by a variable gain amplifier a2 after passing an input resistor r3, then converted into a digital value in an analog-to-digital converter 10, and consequently inputted to the calculation controlling circuit 8.
In the polychrometer 1 constructed as above, the gain of the signal processing circuit 7 is switched in accordance with an amount of an incident light in order to ensure a wide measurement range required for a spectral luminometer. To this end, two kinds of feedback resistors r1, r2 are provided for each amplifier a1 and three kinds of feedback resistors r4, r5, r6 are provided for the variable gain amplifier a2. These feedback resistors r1, r2, r4, r5, r6 are controllably switched by changeover switches s1, s2, s4, s5, s6 provided in series in response to a control signal from the calculation controlling circuit 8 to switch the gains. A ratio of resistance values of the feedback resistor r1 and r2 is, for example, 1:8. A ratio of resistance values of the feedback resistors r4, r5, r6 is, for example, 1:2:4.
Accordingly, in this signal processing circuit 7, the respective gains can be switched to become substantially twofold; a ratio of a minimum gain G=1 to a maximum gain G=6 is about 1:32; and each gain is applied to incident light intensities between a full-scale incident light intensity If(G) and a full-scale incident light intensity If (G-1) of the gain one below the former one as shown in TABLE-1 below.
TABLE 1Gain Number123456ResistanceR1 * R4R1 * R5R1 * R6R2 * R4R2 * R5R2 * R6to beSelectedGain12481632Incident lightI ≦ If1If1 <If2 <If3 <If4 <If5 <Intensity RangeI ≦ If2I ≦ If3I ≦ If4I ≦ If5I ≦ If6
However, if the resistance value is switched in a stepwise manner as described above, input/output relationships at the respective gains become discontinuous and cannot be linear as shown in FIG. 18 because of the deviation of the resistance values from nominal values. Accordingly, outputs by the respective gains need to be corrected by their gain ratios in order to obtain linear input/output relationships over the entire dynamic range (measurement range). For instance, using a ratio of outputs obtained by measuring the same incident light intensity in accordance with two adjacent gains as a correction value, a corrected output can be obtained by multiplying an actual measurement value by this correction value. In order to improve a measurement precision, it is desirable to measure the incident light intensity using higher gains approximate to, but not exceeding the full scale.
On the other hand, silicon photosensors in which the amplifiers a11 to a1n for current-to-voltage conversion are individually provided for the respective photodiodes d1 to dn as described above have a sufficiently linear input/output characteristic in a current short-circuit mode. However, electric charge storing sensor arrays of a self scanning type such as CCD have come to be frequently used as the light receiving sensor array 4 of the polychrometer 1 in recent years since they need not have such a processing circuit for each wavelength and is easy to miniaturize. Such CCD have an S-shaped input/output relationship due to its characteristics such as the saturation of a photoelectrically converting characteristic as shown in FIG. 19 and, hence, has a problem of not having linearity normally required for measuring apparatuses.
Accordingly, in the case of using such a light receiving sensor array 4 in a measuring apparatus such as a spectral luminometer, the aforementioned discontinuity and non-linearity such as an S-shaped characteristic have to be corrected by a certain method. Regardless of whether such a correction is made using a look-up table or functional approximation, output data at a plurality of different incident light intensities are necessary as base data. Particularly in order to obtain precise linearity required for a measurement range of a luminometer extending over 106, measurement data at a multitude of incident light intensities within the measurement range are necessary since outputs of the luminometer are largely deviated from a straight line in a low illuminance area and a high illuminance area. A correction value is calculated from the obtained data and a sensor output is obtained by correcting a measurement value by the correction value at the time of an actual measurement. The non-linearity may differ from element to element and from pixel to pixel even in one element. Thus, for a highly precision measurement, correction needs to be made for each element or for each pixel.
Upon obtaining the data for correction, it has been a conventional practice to adjust the intensity of light beams from a white light source to be incident on a measuring apparatus to be corrected. A white light is used because beams of different wavelengths are incident on the respective pixels of the light receiving sensor array 4. Specifically, the incident light intensity is adjusted as follows:    1. Change a distance from the light source.    2. Change a drive voltage for the light source.    3. Insert an ND filter as disclosed in Japanese Unexamined Patent Publication NO. 2002-174551.
The technique 1 of changing the distance from the light source is such that, taking advantage of the fact that a light amount is in inverse proportion to the square of the distance, the measuring apparatus to be corrected is placed on a bench, the distance is changed, and levels of output signals at the respective distances are measured to obtain a correction value.
The technique 2 of changing the drive voltage for the light source is such that a drive voltage or current to be applied to a light emitting element facing the measuring apparatus to be corrected is changed, and levels of output signals at the respective current values or voltage values to obtain a correction value.
Further, the technique 3 of inserting the ND filter is such that the ND filter for light attenuation is inserted between the measuring apparatus to be corrected and the light emitting element, and levels of output signals at light attenuation rates are measured using filters having different light attenuation rates, thereby obtaining a correction value.
Accordingly, any of the known techniques has problems of necessitating a large facility and requiring considerable labor and time for adjustment. Thus, although correction needs to be made for each element as described above, it is a present situation that some of the elements are sampled and a representative correction value obtained from the sampled elements is set.
Further, in such a construction provided with a multitude of pixel sensors such as a spectral luminometer or a spectral colorimeter, it is desirable to switch the gain at an earliest stage of the signal processing in view of a S/N ratio at a low incident light intensity. In the construction in which the amplifiers al are provided for the respective pixel sensors as shown in FIG. 17, gains differ for the respective pixel sensors of the light receiving sensor array 4. Thus, the above correction needs to be made for each channel, i.e. for each wavelength.
Since the intensity of the light source and the sensitivity of the light receiving sensor array 4 also depend on wavelength, measurement conditions need to be set for the respective pixel sensors for receiving a measurement light separated according to wavelength, which leads to huge labor and time for correction. In other words, if a white light for correction is caused to be incident on a measuring apparatus to be corrected, divided light beams are incident on the respective pixels, whereby the illuminances and the signal outputs of the respective pixels largely differ by being influenced by spectral characteristics of optical elements such as diffraction gratings and CCD. Thus, measurements need to be made at a larger number of incident light intensities in order to illuminate the respective pixels with illuminances suitable for correction.
Further, in order to cover a wide dynamic range and ensure a proper S/N ratio in the low luminance area, the measurement is conducted by extending an electric charge storing time in the CCD. In the case of elements whose non-linearity differs depending on the electric charge storing time, a measurement data at a necessary illuminance level is required for the respective integration times. This disadvantageously leads to even more labor and time required to measure a non-linearity correction.